Aqueous solutions containing surfactants and sometimes so-called builder-substances, such as an alkali metal hydroxide or an alkali metal carbonate, complexing agents, and organic and inorganic corrosion inhibitors are used in industrial degreasing and cleaning operations, particularly in the degreasing and cleaning of metal surfaces. Quite frequently these solutions also contain silicates and/or phosphates. However, for the electrolytic cleaning or spray cleaning of machined parts, certain products can be used that do not contain either silicates or phosphates. For alkaline cleaners, especially for products formulated for electrolytic degreasing, the addition of a surfactant is often omitted. During use, cleaning solutions become contaminated with mineral oil and/or natural oils, as well as with emulsifiers, corrosion inhibitors, and other oily additives. The presence of these contaminants, which are emulsified in the solution by the emulsifier content of the oil and by any surfactants present in the cleaning solution, may cause a decrease in the effectiveness of the aqueous solution, and eventually, will render it inoperative. The solutions must then be replaced. If the spent solutions are alkaline, they must first be neutralized and oils contained in them must be removed in accordance with existing regulations before the solutions can be discarded. Neutralization alone will generally not result in adequate oil separation, i.e. neutralization will not usually break the emulsion. The emulsion can be broken by several methods, e.g. by lowering the pH of the solution to about 1; by treating the solution with salt; or by adding flocculating agents, such as aluminum or iron salts, in the acid range, followed again by neutralization. The components of the cleaning solution are of course lost when the solution, now free of oil, is discarded. Also, treatment of the waste water containing the discarded cleaning solution is generally required. Therefore, it is clearly advantageous to regenerate such spent cleaning solutions so that they can be re-used.
The regeneration of aqueous degreasing solutions can be achieved according to known methods either by means of ultrafiltration or by using centrifugal separators. When using centrifugal separators, only those degreasing solutions can be treated in which the oils and contaminants do not form stable emulsions. In addition, this process, like ultrafiltration, requires a considerable quantity of equipment, is time consuming, and is often energy intensive, making such regeneration processes relatively expensive. The reason why such processes have high equipment requirements is that in many instances the contaminants consist not only of organic substances but also of particles that are not soluble in water.